1. Field of the Invention
The present invention relates to a method for producing an alignment mark, a method for producing a resist mask, a method for transferring a pattern, a method for producing a substrate provided with identification marks, and a method for manufacturing a ceramic substrate for use in a magnetic head.
2. Description of the Related Art
A ceramic substrate for use in a magnetic head is laminated with a magnetic thin layer and the like, and is eventually divided into a plurality of chips. Each of the chips is incorporated into a magnetic head. For the purpose of identifying what substrates the individual chips originate from, it is convenient to record identification information in a prescribed portion on a substrate. For the purpose of identifying what portion of a substrate a chip originates from, it is convenient to allocate unique information to the individual portions (corresponding to the individual chips) of a substrate.
Conventionally, as for the methods for recording the above-described sets of identification information, a method has been widely used in which the sets of identification information are engraved on the surface of a substrate by using a laser marker. According to this method, however, a portion of the substrate surface is melted and scattered by the laser irradiation. As a result, there is a problem in that the engraved portion becomes a source of dust. In the case of a substrate which absolutely must be clean, as in the case of a substrate for use in a magnetic head, dust generation has to be avoided.
Accordingly, a method has been investigated in which identification information is recorded by etching the substrate surface by use of a photoresist mask which has been subjected to patterning. In order to make a large number of chips from a substrate, it is required that identification information should be recorded in the locations respectively corresponding to the individual chips on the substrate with a high degree of accuracy. In order to achieve such accuracy, it is necessary that the alignment of the patterns defining identification information in relation to the substrate is performed with a high degree of accuracy, and accordingly the formation of the alignment marks for alignment on the substrate is required.
In the manufacturing processes of a semiconductor integrated circuit, a variety of thin films are deposited and subjected to etching by the photolithography technique for microfabrication.
Now, a case is considered in which alignment of the interconnection pattern of an upper level performed in relation to the interconnection pattern of an underlying level. In such a case, after the metal film for the upper level interconnections is deposited, a photoresist layer is formed over the metal film. Then, in order to impart a predetermined interconnection pattern to the photoresist layer, the position of a photomask is adjusted in a light exposing apparatus. This position adjustment is performed on the basis of the alignment marks formed on the substrate.
The alignment marks are constituted by the concave portions and convex portions formed on the underlying layers including the substrate surface and the other layers. The alignment marks are formed, for example, from a metal film during the step of patterning the metal film into the lower level interconnections. Such alignment marks need not be finally removed, and usually, often are allowed to remain on the substrate.
In the case where an alignment mark is formed directly on the substrate surface, it is possible that the substrate surface is subjected to etching to form a concave alignment mark on the substrate surface. In the case where identification information is recorded on the substrate surface, however, it is not preferable that there are unnecessary concavities and convexities other than identification information. Accordingly, it is desirable that the alignment mark is neatly removed from the substrate after the alignment process has been completed.
On the basis of the above descriptions, conventional methods for producing an alignment mark are described below. Description will now be made of a conventional method for producing an alignment mark with reference to the drawings.
In the first step, as FIG. 1A shows, a metal film 12 is deposited on the substrate 10, and then a photoresist layer 14 is formed over the metal film 12. In the next step, the pattern defining an alignment mark is transferred on the photoresist layer by patterning the photoresist layer using the photolithography technique. Specifically, the metal film 12 is coated with the positive photoresist layer 14 (the thickness is, for example, 1 μm) by the spin coat method, and then the prescribed region on the photoresist layer is exposed to radiation through a photomask which defines the pattern for the alignment mark. The irradiated portion (the exposed portion) on the photoresist layer 14 is made photolytically to be soluble in a developer. Then, as shown in FIG. 1B, the exposed portion is dissolved in the developer in the developing process, so that a resist mask 15 is formed that is constituted from the nonexposed portion of the photoresist layer.
In the next place, as FIG. 1C shows, an alignment mark 13 is formed from the metal film 12 by etching the portion of the metal film 12 which is not covered with the resist mask 15. The shape and location of the alignment mark 13 are specified by the resist mask 15.
In the next step, the resist mask 15 is removed, as FIG. 1D shows, and then a second photoresist layer (the thickness is, for example, 1 μm) 16 is formed so as to cover the alignment mark 13, as FIG. 1E shows.
As FIG. 1F shows, a prescribed portion of the second photoresist layer 16 is exposed and developed by the photolithography technique, and thus the patterning of the second photoresist layer 16 is performed. In this way, a resist mask 17 is formed which has an opening portion 18 defining identification information.
As FIG. 1G shows, by etching the region on the substrate surface which is not covered with the above described resist mask 17, a concave portion 20 is formed which displays identification information on the surface of the substrate 10. After this process, the resist mask 17 is removed, as FIG. 1H shows.
In the case where identification information is recorded on the surface of the substrate 10 according to the above described methods, the alignment mark 13 still remains in the region on the substrate surface where no identification information is recorded.
If there is an unnecessary convex portion such as the alignment mark 13 in a substrate provided with identification information as described above, many problems occur including the flatness of the multi-layer structure to be formed on the substrate 10 being degraded. In the case where identification information is engraved on the back side of the substrate 10, if the above described alignment mark 13 remains on the back side of the substrate 10, there is also a concern that some problems will occur when the substrate is transported, and the substrate is fixed to the holders (the members for holding a substrate) in a variety of instruments.
For the purpose of overcoming such problems, it is preferable that the alignment mark 13 is removed from the substrate 10 at the time that the alignment process ends and the alignment mark 13 becomes unnecessary. However, in order to remove the alignment mark 13 formed from the metal film 12 from the substrate 10, a process is necessary in which the alignment mark 13 is completely etched away by use of an appropriate etchant.
Etching of metal is also conducted when the alignment mark 13 is formed from the metal film 12. In this case, if the patterning of the metal film 12 is needed to be performed with a high accuracy, it is necessary that the etching is conducted by using, for example, a dry etcher. However, it is difficult to carry out the dry etching over a wide surface area of a substrate with a uniform rate, and accordingly, it is necessary to adopt the etching conditions that allow overetching to occur in order to prevent insufficient etching from occurring. In the case where such conditions are adopted, as FIG. 2 shows, steps A are formed in the edge portions of the alignment mark 13 in some cases. Such steps A persist after the removal of the alignment mark 13, and thus possibly cause the degradation of the read accuracy when identification information is optically read.